1. Field of the Invention
The invention presented herein relates to electrosurgical units and, more specifically, to circuitry for providing automatic control of the output power at a selected level over the usual impedance range of electrically conductive tissue encountered when using such units.
2. Description of the Prior Art
A wide variety of electrosurgical units for generating various high frequency output waveforms for electrosurgical procedures are known in the art. A selected waveform output from an electrosurgical unit is applied to a patient by the use of an active electrode placed in contact with the patient at the point where a desired surgical procedure is to be carried out on tissue with current return provided via the patient and a plate or return electrode positioned in electrical contact with the patient. The active electrode provides a small area contact with the tissue of the patient to cause the current density at such contact to be high enough to generate heat sufficient to accomplish the desired surgical procedure.
Since the impedance of the current path is a function of the different electrically conductive tissue types that may be encountered, the power delivered by the electrosurgical unit will vary dependent on the tissue encountered when the unit is set to provide a selected desired output. It has been a common practice to merely match the output impedance of the electrosurgical unit to the median of the expected impedance range of the tissue. This method is not acceptable since the output power decreases as the impedance varies from the median or center designed value. In order that adequate power be provided at both low or high impedance tissue with such method, it is necessary to provide for a power level at the median of the impedance range which may be excessive resulting in unnecessary tissue damage.
U.S. Pat. No. 3,601,126 points out the power level problem involved due to the range of impedance that is encountered when using an electrosurgical unit. The patent purports to solve this problem by monitoring the load current from the secondary of an output transformer by the use of a square law detector, the output of which is compared with a reference voltage level with the difference that is detected being used to vary the output of the electrosurgical unit. Such an arrangement, while providing a constant load current, produces in a linear increase in power to the load as the load impedance increases rather than providing a constant power output. Further, by sensing current flow on the secondary side of the output transformer, a degradation of high frequency isolation is presented.